As known to those skilled in the art, a heating element consists of an area characterised by a low electrical resistance referred to as the “cold zone” and by an area having a much higher electrical resistance that constitutes the actual heating part.
In one heating element design, two cold zones are associated with the hot zone by being connected in series so that the intensity of the electrical current that flows through each of the zones is identical.
The function of the cold zones is to connect the current supply obtained from an external source and to distribute the current uniformly to the terminals of the heating part.
The reader is reminded that power equals resistance times current squared and that the flow of current produces a significant temperature rise in the area which has a higher electrical resistance and is referred to as the hot zone. Conversely, given the fact that the cold zones have the lowest possible electrical resistance, minimal thermal power is released from the electrical connections.
Also, because resistance equals resistivity times the length of the conductor divided by its cross-sectional area, it is possible to modify resistance values by varying any of the above-mentioned parameters Resistivity, length, cross-sectional area.
In one embodiment of thin-layer heating elements using, for example, a vacuum deposition technique, one can employ at least two materials of different resistivity, each of them being deposited through specific masks in order to successively constitute a (hot zone) heating part in the form of a track and one or more collectors or drains. The two materials are selected depending on their intrinsic resistivity, whereas their cross-sectional areas are determined depending on the conductance values required for the cold zones and the resistance value required for the heating part.
If the heating coating does not make it possible to obtain a sufficiently high electrical resistance by adjusting the thickness of said coating, it becomes necessary to alter the length of the resistance deposited in the form of tracks in order to consequently increase the distance over which the current flows.
As stated above, a second highly conductive material is deposited at the ends of the resistances in order to act as a drain. One can cite, for example, the disclosures in Patents JP7226301 and JP8124707.
In another embodiment, if the deposited resistance is not in the form of a track, electrical connections can be made either side of the resistive coating through a conductive coating produced in the form of strips known as drains. Such a solution is disclosed in Documents WO0158213, WO03095251 and U.S. Pat. No. 4,543,466. As disclosed in Document WO0158213, the heating layer is made of a transparent conductive material and it is associated with a highly conductive layer of silver in order to fulfil the drain function without any temperature rise.
As a result of this state of the art, producing a heating element by depositing thin layers involves two stages:                a first stage to deposit the electrical resistance through a mask in the case of a resistance in the form of tracks or to deposit it directly onto the entire surface of the substrate. Note that tracks may also be fabricated by selectively removing the resistive deposit;        a second stage to deposit another coating through another mask to produce drains.        
It is therefore necessary to use two different materials and to manipulate a mask between the two deposition processes.
This state of the art is illustrated schematically in FIG. 1 which shows a heating element produced in two stages. The heating element consists of a substrate (C) made of an electrically insulating material such as a ceramic, glass or plastic etc. A slightly conductive coating (B) is deposited over the entire surface of substrate (C). A mask is positioned on coating (B) in such a way that, for example, it does not cover its ends so as to make it possible to deposit a second highly conductive coating in order to produce drains (A) for connecting an electrical power source such as a generator (G).